ABSTRACT: Frank Easterbrook’s 1984 article, The Limits of Antitrust, did not focus on public antitrust enforcement. Nevertheless, it expressed the kind of antitrust thinking that led the Antitrust Division of the U.S. Department of Justice, around that same time, to shift its resources to cartel prosecutions and away from big monopolization cases. The Microsoft case, filed in 1998, broke this pattern. I argue that the Division made this exception, and ultimately achieved a partial victory in the courts, because the relatively new economic theory of network effects seemed to make the filters Easterbrook proposed in 1984 less applicable in high technology markets like the ones in which Microsoft competed. In this essay, I return to Easterbrook’s filters and consider whether they offer a different perspective on the Division’s decision to sue and the courts’ eventual resolution of the case.